There is a present need to decrease carbon dioxide (CO2) emissions from industrial facilities. Over the years, a number of electrochemical processes have been suggested for the conversion of CO2 into useful products. Processes for CO2 conversion and the catalysts for them are discussed in U.S. Pat. Nos. 3,959,094; 4,240,882; 4,523,981; 4,545,872; 4,595,465; 4,608,132; 4,608,133; 4,609,440; 4,609,441; 4,609,451; 4,620,906; 4,668,349; 4,673,473; 4,711,708; 4,756,807; 4,818,353; 5,064,733; 5,284,563; 5,382,332; 5,457,079; 5,709,789; 5,928,806; 5,952,540; 6,024,855; 6,660,680; 6,987,134 (the '134 patent); U.S. Pat. Nos. 7,157,404; 7,378,561; 7,479,570; U.S. Patent Application Publication No. US 2008/0223727 A1 (the '727 publication); and papers reviewed by Hori (Modern Aspects of Electrochemistry, 42, 89-189, 2008) (“the Hon review”), Gattrell, et al. (Journal of Electroanalytical Chemistry, 594, 1-19, 2006) (“the Gattrell review”), DuBois (Encyclopedia of Electrochemistry, 7a, 202-225, 2006) (“the DuBois review”), and the papers Li, et al. (Journal of Applied Electrochemistry, 36, 1105-1115, 2006), Li, et al. (Journal of Applied Electrochemistry, 37, 1107-1117, 2007), and Oloman, et al. (ChemSusChem, 1, 385-391, 2008) (“the Li and Oloman papers”).
Generally an electrochemical cell 10 contains an anode 50, a cathode 51 and an electrolyte 53 as indicated in FIG. 1. The devices can also include a membrane 52. Catalysts are placed on the anode, and or cathode and or in the electrolyte to promote desired chemical reactions. During operation, reactants or a solution containing reactants is fed into the cell via anode reactant manifold 54 and cathode reactant manifold 55. Then a voltage is applied between the anode and the cathode, to promote an electrochemical reaction.
When an electrochemical cell is used as a CO2 conversion system, a reactant comprising CO2, carbonate or bicarbonate is fed into the cell. A voltage is applied to the cell, and the CO2 reacts to form new chemical compounds. Examples of cathode reactions in the Hori review include:CO2+2e−→CO+O2 2CO2+2e−→CO+CO32−CO2+H2O+2e−→CO+2OH−CO2+2H2O+4e−→HCO−+3OH−CO2+2H2O+2e−→H2CO+2OH−CO2+H2O+2e−→(HCO2)−+OH−CO2+2H2O+2e−→H2CO2+2OH−CO2+6H2O+6e−→CH3OH+6OH−CO2+6H2O+8e−→CH4+8OH−2CO2+8H2O+12e−→C2H4+12OH−2CO2+9H2O+12e−→CH3CH2OH+12OH−2CO2+6H2O+8e−→CH3COOH+8OH−2CO2+5H2O+8e−→CH3COO−+7OH−CO2+10H2O+14e−→C2H6+14OH−CO2+2H++2e−→CO+H2O, acetic acid, oxalic acid, oxylateCO2+4H++4e−→CH4+O2 where e− is an electron. The examples given above are merely illustrative and are not meant to be an exhaustive list of all possible cathode reactions.
Examples of reactions on the anode mentioned in the Hon review include:2O2−→O2+4e−2CO32−→O2+2CO2+4e−4OH−→O2+2H2O+4e−2H2O→O2+4H++4e−
The examples given above are merely illustrative and are not meant to be an exhaustive list of all possible anode reactions.
In the previous literature, catalysts comprising one or more of V, Cr, Mn, Fe, Co, Ni, Cu, Sn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Ir, Pt, Au, Hg, Al, Si, In, Sn, Tl, Pb, Bi, Sb, Te, U, Sm, Tb, La, Ce, and Nd have all shown activity for CO2 conversion. Reviews include Ma, et al. (Catalysis Today, 148, 221-231, 2009), Hori (Modern Aspects of Electrochemistry, 42, 89-189, 2008), Gattrell, et al. (Journal of Electroanalytical Chemistry, 594, 1-19, 2006), DuBois (Encyclopedia of Electrochemistry, 7a, 202-225, 2006) and references therein.
The results in the Hori review show that the conversion of CO2 is only mildly affected by solvent unless the solvent also acts as a reactant. Water can act like a reactant, so reactions in water are different than reactions in non-aqueous solutions. But the reactions are the same in most non-aqueous solvents, and importantly, the overpotentials are almost the same in water and in the non-aqueous solvents.
Zhang, et al. (ChemSusChem, 2, 234-238, 2009) and Chu, et al. (ChemSusChem, 1, pages 205-209, 2008) report CO2 conversion catalyzed by an ionic liquid. Zhao, et al. (The Journal of Supercritical Fluids, 32, pages 287-291, 2004) and Yuan, et al. (Electrochimica Acta 54, pages 2912-2915, 2009) report the use of an ionic liquid as a solvent and electrolyte, but not a co-catalyst, for CO2 electroconversion. Each of these papers is incorporated by reference. Catalyst Today, Volume 48, pages 189-410 November 2009 provides the proceedings of the 10th international conference on CO2 utilization. These pages are incorporated by reference. The catalysts have been in the form of either bulk materials, supported particles, collections of particles, small metal ions or organometallics. Still, according to Bell (A. Bell, Ed., Basic Research Needs, Catalysis For Energy, U.S. Department Of Energy Report PNNL17712, 2008) (“the Bell Report”), “The major obstacle preventing efficient conversion of carbon dioxide into energy-bearing products is the lack of catalyst” with sufficient activity at low overpotentials and high electron conversion efficiencies.
The overpotential is associated with lost energy of the process, and so one needs the overpotential to be as low as possible. Yet, according to the Bell Report, “Electron conversion efficiencies of greater than 50 percent can be obtained, but at the expense of very high overpotentials”.
The '134 patent also considers the use of salt (NaCl) as a secondary “catalyst” for CO2 reduction in the gas phase, but salt does not lower the overpotential for the reaction.
A second disadvantage of many of the catalysts is that they also have low electron conversion efficiency. Electron conversion efficiencies over 50% are desirable for practical catalyst systems.
The examples above consider applications for CO2 conversion, but the present invention overcomes limitations of other systems. For example some commercial CO2 sensors use an electrochemical reaction to detect the presence of CO2. At present, these sensors require over 1-5 watts of power, which may be too high for portable sensing applications.
The present invention also considers, for example, new methods to produce or electrochemically react formic acid. Other methods of generating formic acid are discussed in U.S. Pat. Nos. 7,618,725; 7,612,233; 7,420,088; 7,351,860; 7,323,593; 7,253,316; 7,241,365; 7,138,545; 6,992,212; 6,963,909; 6,955,743; 6,906,222; 6,867,329; 6,849,764; 6,841,700; 6,713,649; 6,429,333; 5,879,915; 5,869,739; 5,763,662; 5,639,910; 5,334,759; 5,206,433; 4,879,070; and 4,299,891. These processes do not use CO2 as a reactant.
Formic acid can be used, for example, in fuel cells. It has been shown that the oxidation reaction of formic acid in a fuel cell can be poisoned by organic acids such as acetic acid, as well as by methyl formate or methanol. See, for example, Masel, et al., U.S. Pat. No. 7,618,725 (Low Contaminant Formic Acid Fuel For Direct Liquid Fuel Cell).
Another benefit of the present invention is that it can suppress undesirable side reactions, such as the generation of hydrogen gas from the electrolysis of water in an electrochemical cell. This hydrogen evolution reaction (HER) can reduce the electron conversion efficiency of a desired reaction, and in some instances may present a safety hazard from the buildup of potentially explosive hydrogen gas. In Monsanto U.S. Pat. No. 4,207,151 (Electrohydrodimerization Process Improvement And Improved Electrolyte Recovery Process), Franke, et al. described inhibiting formation of hydrogen at the cathode surface by adding to the aqueous solution a nitrilocarboxylic acid. One such nitrilocarboxylic acid cited is the complexing agent ethylenediaminetetraacetic acid (EDTA). The patent also discloses that the “generation of hydrogen at the cathode is even more significantly inhibited by including in the electrolysis medium a boric acid, a condensed phosphoric acid or an alkali metal or ammonium salt thereof,” such as ammonium triphosphate. The process improvement method also discloses incorporating at least a small amount of quaternary ammonium cations in the aqueous phase as a “directive salt”, in order to improve the phase partition extraction efficiency for separating the desired product. “In general, there need be only an amount sufficient to provide the desired hydrodimer selectivity (typically at least about 75%) although much higher proportions can be present if convenient or desired.” Quaternary ammonium salts can also be used in the process as conductive salts to provide the desired conductivity of the cell electrolyte. A more detailed history of the development of this process is provided by D. E. Danly, “Development and Commercialization of the Monsanto Electrochemical Adiponitrile Process,” Journal of the Electrochemical Society, October 1984, pages 435C-442C. This paper indicates that the hydrogen suppression by the addition of the nitrilocarboxylic acid EDTA was accomplished by chelating Fe and Cd anode corrosion products before they could reach the cathode. The paper stated that, “In the absence of EDTA, hydrogen evolution at the cathode increased over a day's operation to the point where it represented greater than 10% loss in cathodic current efficiency.”
Rezaei and Taki have recently shown that the quaternary amine tetrabutylammonium hydrogen sulfate (TBAHS) can increase the hydrogen overpotential for the hydrogen evolution reaction (HER) in a lead acid battery that uses Pb—Sb—Sn positive and negative electrode grids. (Behzad Rezaei and Mahmood Taki, “Effects of tetrabutylammonium hydrogen sulfate as an electrolyte additive on the electrochemical behavior of lead acid battery,” J. Solid State Electrochem. (2008) 12:1663-1671). Water loss has been high in such batteries because antimony from the positive grid can migrate through the sulfuric acid electrolyte solution and be deposited on the negative plate, where it diminishes the overpotential for hydrogen evolution from the electrolysis of water. TBAHS was selected as a possible electrolyte additive material that might be able to withstand the sulfuric acid electrolyte. Rezaei, et al., similarly investigated ammonium hydrogen sulfate salts of a primary, a secondary, and a tertiary amine, as well as the “aromatic quaternary amine” 1-butyl-3-methylimidazolium hydrogen sulfate (BMIM HS). The results were somewhat inconsistent, particularly for the BMIM HS. Also, the addition of these materials to the battery electrolyte was found to increase the grid corrosion rate. (Behzad Rezaei, Shadpour Mallakpour, and Mahmood Taki, “Application of ionic liquids as an electrolyte additive on the electrochemical behavior of lead acid battery,” J. of Power Sources, 187 (2009) 605-612).
Substituted benzaldehydes were studied for suppressing hydrogen evolution to reduce water loss during cycling by Dietz, et al., “Influence of benzaldehydes and their derivatives as inhibitors for hydrogen evolution in lead/acid batteries,” Journal of Power Sources, 53, pages 359-365 (1995).
The addition of succinic acid to the electrolyte of a fuel cell was found to greatly increase the hydrogen evolution overpotential and reduce hydrogen generation in the investigation by Lee, et al., “Study on Suppression of Hydrogen Evolution Reaction for Zinc/Air Fuel Cell,” Materials Science Forum, Vols. 539-543, pages 1427-1430 (2007).
One recent paper mentions hydrogen evolution from trace amounts of water as a side reaction during electrodeposition of metals from the deep eutectic solvent Ethaline 200 (choline chloride with ethylene glycol). See Haerens, et al., “Electrochemical decomposition of choline chloride based ionic liquid analogues,” 2009 Green Chemistry 11 (9), pages 1357-1365.
The quaternary amine salt choline dihydrogen phosphate has recently been investigated as a possible solid state proton exchange membrane for applications such as fuel cells and sensors. See, for example, Yoshizawa-Fujita, et al., “A new class of proton-conducting ionic plastic crystals based on organic cations and dihydrogen phosphate,” 2007 Electrochemistry Communications 9 (5), pages 1202-1205, and Cahill, et al., “Investigation of proton dynamics and the proton transport pathway in choline dihydrogen phosphate using solid-state NMR,” 2010 Physical Chemistry Chemical Physics 12 (20), pages 5431-5438.
In light of the above, there still exists a need for cost effective methods to suppress undesired reactions such as the hydrogen evolution reaction in applications such as electrochemical cells, fuel cells, and sensors, while simultaneously enhancing the rate or yield of the desired reaction(s). In particular there is a need to suppress the hydrogen evolution reaction using additives that do not contain carboxylate groups, since carboxylic acids and their salts inhibit desired reactions such as formic acid electrooxidation or carbon dioxide conversion.